An intervertebral disc is a soft tissue compartment connecting the vertebrae bones in a spinal column. Each healthy disc consists of two parts, an outer annulus fibrosus (hereinafter “the annulus”) and an inner nucleus pulposus (hereinafter “the nucleus”). The annulus completely circumscribes and encloses the nucleus. The annulus is connected to an adjacent associated pair of vertebrae by collagen fibers. The intervertebral disc is an example of a soft tissue compartment adjoining first and second bones (vertebrae) having an initial height and an initial width. Other joints consisting of a soft tissue compartment adjoining at least first and second bones having an initial height and an initial width include the joints of the hand, wrist, elbow, shoulder, foot, ankle, knee, hip, etc.
Typically, when a disc is damaged, the annulus ruptures and the nucleus herniates. Discectomy surgery removes the extruded nucleus, leaving behind the ruptured annulus. The ruptured annulus is, by itself, ineffective in controlling motion and supporting the loads applied by the adjacent pair of vertebrae. With time, the disc flattens, widens, and bulges, compressing nerves and producing pain. Uncontrolled loads are transmitted to each vertebra. Each vertebra tends to grow wider in an attempt to distribute and compensate for higher loads. When a vertebra grows, bone spurs form. The bone spurs further compress nerves, producing pain.
A variety of expandable intervertebral devices are disclosed in the art to replace the intervertebral disc. Such devices are implanted intermediate an adjacent pair of vertebrae, and function to assist the vertebrae. These devices do not assist the intervertebral disc. In fact, in many cases the disc is removed.
Prior art intervertebral devices are either static or dynamic. A static intervertebral device eliminates motion. Static devices are generally square, rectangular, trapezoidal, or box shapes that are immobile. Static devices replace the disc to facilitate bone fusion. The insertion of a static device requires near-total removal of the disc. An adjacent pair of vertebrae ordinarily is contoured to the static device and a bone graft. A static device temporarily maintains the vertebrae immobilized until the bone graft heals. Static devices may, on insertion, initially expand, but their final state is immobile. Core elements with the threads on one portion reversed or oppositely wound from threads on another portion have been frequently utilized to expand immobilization (fusion) devices.
Dynamic devices are configured to be capable of moving. Inserting a dynamic device, such as a total disc prosthesis, requires a near-total removal of disc tissue. A dynamic device ordinarily is inserted to contour to the vertebral bones without a bone graft. Usually the vertebral bones are contoured to the dynamic device. Round, curved, or circular-shaped devices inserted after removing disc tissue or vertebral bone tend to migrate in the intervertebral disc space or subside within the vertebral bone. Dynamic devices are often permanent devices that replace a disc, connect vertebral bones together, and allow movement. Dynamic devices initially may expand. Also, their final state can be mobile.
Other dynamic devices require partial removal of disc tissue. These devices can be inserted within the interior (nucleus) of an intervertebral disc and contour to the vertebral bones. Nucleus devices are generally smaller than devices used as a total disc prosthesis. Nucleus devices often are single part-locking mechanisms. Fixation generally is not used and the device typically migrates within the disc space or subsides in vertebral bones. Other dynamic devices do not have a solid-bearing surface but include liquid or gas.
Other devices and methods function to patch or seal a disc without substantially supporting the vertebrea. Inserting these devices requires the removal of disc tissue. These devices are usually added to the annulus, which causes widening of the annulus, and the device increases the risk of contacting the nerves of the spinal column when the disc is compressed. Still other devices can form a physical barrier with the annulus in order to function, where such a barrier positioned within the annulus can prevent the annulus from healing. Still other devices change the material property of the disc.
Therefore, there remains a need in the art for improved devices and methods for treating injuries, deformations, or other defects in any of the intervertebral discs of the spine.